1. Field of the Invention
The present invention relates to a ranging system and a method thereof. More particularly, the present invention relates to a ranging system to accurately measure a distance between two devices and reduce power consumption and channel occupancy in transmitting a transmission pulse and a method thereof.
2. Description of the Related Art
In the IEEE 802.15.4 standards, a full function device (FFD) and a reduced function device (RFD) are suggested. In the FFD, wireless communication between two devices may occur without a carrier; but in the RFD, wireless communication between two devices requires a carrier.
Figuring out a location by communicating with another device using FFD and RFD methods is called localization. All the functions of a physical layer to measure a relative distance between devices upon executing the localization function are called ranging technology. The ranging technology offers a key reference to utilize information obtained through communications by efficiently forming networks.
There are a single trip method and a round trip method for the ranging technology.
A single trip method measures how long it takes to transmit an RF signal using an absolute time between a transmission device transmitting the RF signal and a reception device, and calculates a distance between the two devices using the time.
A round trip method calculates a distance between two devices using how long it takes for the RF signal to go and return between the two devices, if an RF signal transmitted from a transmission device is processed in a reception device and re-received to the transmission device. In the case of the round trip method, a round trip time of the RF signal includes a time for processing the RF signal in the reception device and the processing time can be simply calculated according to the device features. Accordingly, if the processing time is subtracted from the round trip time of the RF signal and then multiplied by ½, the result is the one-way moving time of the RF signal between the two devices. So, by multiplying this time by the speed of light, the distance between the two devices may be calculated.
A certain signal delay occurs in the RF signal going and turning between the two devices. To measure the distance between the two devices considering the delay, each device includes a transmission pulse generator 11, a multiplier 13, a reference pulse generator 15, a time calculator 17, an overlap detector 19 and a distance calculator 21 as shown in FIG. 1.
The transmission pulse generator 11 generates a transmission pulse at a preset frequency, for example, at 2.5125 MHz, to transmit to a reception device.
The multiplier 13 multiplies the transmission pulse generated from the transmission pulse generator 11 by a chaotic signal source to convert the transmission pulse into a chaotic signal.
The reference pulse generator 15 generates a reference pulse to compare with the transmission pulse which is processed and returned from the reception device. The reference pulse has a certain frequency width different from the transmission pulse. For example, the reference pulse generator 15 generates a reference pulse at 5.125 MHz as shown in FIG. 2A.
The transmission pulse generator 11 and the reference pulse generator 15 generate the transmission pulse and reference pulse at the same time. The transmission pulse is transmitted to the reception device, but the reference pulse is not externally transmitted.
The overlap detector 19 compares the reference pulse in the transmission device and the transmission pulse re-transmitted from the reception device, and detects a region where the reference pulse and the transmission pulse overlap each other. The reference pulse and the transmission pulse have a frequency difference of 0.0125 MHz. Accordingly, if no delay between the reference pulse and the transmission pulse occurs, the reference pulse and the transmission pulse overlap each other at a certain earlier region. However, if a delay occurs between the reference pulse and the transmission pulse, a region where the reference pulse and the transmission pulse overlap each other can be another region instead of the earlier region. The overlap detector 19 detects the region where the reference pulse and the transmission pulse overlap to determine how long the transmission pulse is delayed.
The time calculator 17 counts (1) the number of pulses between the moment when the transmission pulse is transmitted to the reception device and right before the returned transmission pulse and the reference pulse overlap each other, (2) the number of pulses between the moment when the transmission pulse returns to the transmission device and the moment when the returned transmission pulse and the reference pulse overlap each other, and (3) the number of pulses while the returned transmission pulse and the reference pulse overlap each other. Using each of the counted numbers, a transmission time of the transmission pulse moving between the transmission device and reception device is calculated, and a time required to process the transmission pulse in the reception device has to be considered.
The distance calculator 21 calculates a distance between the two devices using the transmission time calculated in the time calculator 17, the speed of light, and the transmission pulse in the reception device.
However, the re-transmitted transmission pulse is a chaotic pulse and is influenced by noise generated during the process of the reception device and multi-path channels. Accordingly, when the shape of each pulse of the transmission pulse is determined using a threshold as shown in FIG. 2, accuracy decreases. Due to inaccuracy of reception of the transmission pulse, a region where the transmission pulse and the reference pulse overlap each other is different every pulse so that the counted number can be different from reality. Therefore, it is difficult to accurately measure the delay and a relative distance between two devices.
Meanwhile, 250 transmission pulses are needed to measure a distance using the transmission pulse, and a large amount of power is consumed to transmit the 250 transmission pulses. In addition, channels are occupied during the transmission of the 250 transmission pulses so that a time for occupying channels is long. For example, when an RF signal is transmitted at 2.5 Mbps, channels are occupied during 400 ns*250*2(round trip)=200 us.
Therefore, there is a need for a method of accurately measuring a distance by accurately determining each pulse of the received transmission pulses. In addition, there is a need for a method of reducing power consumption and channel occupancy when transmitting a transmission pulse.